This invention relates to apparatus for measuring operating parameters of a high voltage power conductor. More particularly, it relates to radio transmitting sensors which are mounted on the transmission lines. The sensors normally derive their power through electromagnetic induction using the magnetic field generated as a result of current flowing through the transmission line conductor. Transmission line sensors of the above type are disclosed in the U.S. patent of Howard R. Stillwell and Roosevelt A. Fernandes entitled Transponder Unit for Measuring Temperature and Current on Live Transmission Lines, No. 4,384,289, issued May 17, 1983, which patent is incorporated herein by reference.
Various power line sensors have been disclosed in the prior art. For example, see U.S. Pat. Nos. 3,428,896, 3,633,191, 4,158,810 and 4,268,818. Sensors of this type and of the improved form disclosed in the above-identified Stillwell and Fernandes patent have been proposed for dynamic line rating of electrical power transmission lines.
The power line sensors available in the prior art measure quantities associated with operation of the overhead line such as voltage on the conductor, current flow in the conductor, conductor temperature and ambient temperature. The information gathered is transmitted to a local ground station. Data from various ground receivers is transmitted to a central control station where the information is analyzed. Based on this analysis, the entire power system is controlled with power being supplied to the various transmission lines in accordance with their measured parameters.
The compact toroidal-shaped sensor modules, disclosed in U.S. Pat. No. 4,384,289, and in the prior copending applications referenced above, contain the sensor electronics, power supply and transmitter circuits, and hardware for attaching the sensors to the transmission lines. Under normal operating conditions, power for the sensor module is derived from the transmission line conductor and converted into regulated supply voltages for the sensor electronics and transmitter.
The module power supply is coupled to the transmission line conductor by a transformer. The line conductor extends through a magnetic core in the module housing and is utilized as a single turn primary winding in the transformer. A secondary winding on the core is connected to power conversion and regulation circuits to supply the required voltages for the module's electronics.
The harsh environment of power transmission lines subjects the sensor module to power supply fluctuations due to a variety of transient line conditions. Under certain conditions, e.g., fault conditions, current flow in the conductor will be far in excess of the current required by the load (i.e., sensor electronics and transmitter).
In the sensor modules described in the copending applications, the voltage supplied to the load is regulated by monitoring the DC output voltage and removing the load from the transformer secondary whenever the DC output voltage is too high. However, when the conductor current is very high, large secondary voltage peaks appear across the unloaded winding as the alternating current in the conductor passes through zero. This may result in damage to power supply circuitry.
The open circuit voltage across the secondary winding can be many hundreds of volts and will depend on the number of turns of the winding. It is desirable to have the sensor module be operable even when current in the conductor is at very low levels. As a result, many turns in the secondary winding are required so that a sufficient DC output voltage can be produced to power the module electronics under low current conditions. Unfortunately, this also results in unmanageable secondary voltages being produced when current is very high. To overcome this problem, rectifier and regulation circuits can be selected to withstand high voltages. These circuits, however, will inhibit operation under low current conditions because of their large forward voltage drop or their requirement for large gate drive power.
In accordance with the present invention, the transmission line sensor module is provided with a shunt regulator that inhibits high voltage induction in the transformer secondary resulting from excess currents flowing in the conductor (transformer primary). The magnetic circuit of the power supply has been altered by providing a secondary core which acts as a magnetic shunt for the primary core.
The primary core in the sensor housing has a relatively large cross sectional area and includes a small air gap which separates two primary core halves. This is necessary to permit the sensor module to be opened and installed around the conductor. The primary core has a relatively high inductance at low levels of current which enables adequate output voltage to be available when minimum current is flowing in the conductor. The secondary core includes a larger air gap and, at low flux densities has high reluctance relative to the main core. The secondary core does not, therefore, impair the voltage output at low currents. The size of the air gap in the secondary core determines at what value of current in the conductor flux is diverted into the secondary core. The secondary core has a larger cross sectional area than the primary core. This inhibits saturation of the secondary core when current values are large and the primary core (excluding the portion linking the winding) becomes saturated.
In the power supply of the present invention, an effective short circuit is placed across the secondary winding when the load voltage exceeds a predetermined value. Current in the winding will be proportional to the reluctance caused by the air gap in the secondary core which acts to limit the rapid saturation of that part of the primary core that links the winding. Current induced in the winding, and associated dissipation, are therefore limited to an acceptable value. However, the total reluctance of the magnetic circuit in a circular path around the winding (including a portion of the primary core and the secondary core) is low enough to permit sufficient induced voltage even with minimum currents.
It is therefore an object of the present invention to provide a transmission line sensor module operable under both the conditions of very high conductor current and relatively low conductor current.
Another object of the invention is to provide a power supply circuit which inhibits excessive output voltages when the transmission line conductor current is high.
A further object of the invention is to provide a power supply circuit which effectively short circuits the power transformer secondary when a predetermined output voltage level is sensed, thereby protecting the load circuitry.
Another object of the invention is to provide a magnetic circuit in a transformer with a shunt magnetic path having a relatively high reluctance to limit induction of secondary voltages and short circuit current in the secondary winding.
Still another object of the invention is to provide a magnetic circuit in a transformer with a relatively high inductance at low flux densities to enable sufficient output voltages to be produced, while having a relatively high reluctance at high flux densities to limit rapid saturation of the magnetic core linking the winding.